1. Field of the Invention
The present invention relates to a sheet transporting device for use in sheet handling apparatus such as copying machines, laser printers and the like. More specifically, the present invention relates to a sheet transporting device capable of delivering sheets between a first transporting means and a second transporting means having different stopping characteristics and starting characteristics, and capable of re-transporting a sheet after the sheet is temporarily held in a state bilaterally straddling the first and second transporting means.
2. Description of the Related Art
The aforesaid type of sheet transporting device is used, for example, when a copying machine with a document feeder disposed on a base unit forms images on a sheet transported from the base unit document feeder. More specifically, in the main unit of the copying machine, the transported transfer sheet is temporarily stopped at a timing roller disposed immediately in front of the transfer portion and then re-transported after a predetermined interval so as to prevent skewing and align the leading edges of the sheet and the image formed on the surface of the photosensitive drum. During the aforementioned temporary stopping time, the sheet often bilaterally straddles the base unit side and the copying machine side depending on the size of the sheet. Thus, the sheet transporting mechanism on the base unit side and the sheet transporting mechanism on the copying machine side, which feeds the transfer sheet received from the base unit side to the timing roller, are temporarily stopped for a predetermined interval after the paper sheet has reached the timing roller and then are re-actuated with a predetermined timing.
If the sheet transporting mechanism within the base unit is connected to and controlled by the various mechanisms within the main unit of the copying machine, these mechanisms and controls become quite complex and expensive, and further make it quite inconvenient to move the copying machine and the like. Therefore, the aforesaid construction and controls may be simplified by operating the sheet transporting mechanism in the base unit by means of a motor independent of the copying machine unit, thereby also allowing convenience in moving the copying machine and the like. In such a case, the timing roller in the copying machine unit and the sheet transporting mechanism disposed anteriorly thereto jointly use the main motor provided in the copying machine unit, and are mutually connected via the main motor and a clutch so as to be respectively actuatable as required. The base unit sheet transporting mechanism, however, is actuated and stopped by switching on and off an independent motor without providing a discoupling structure between said sheet transporting unit and the independent motor of the base unit, thereby simplifying the construction and controls.
The differences between the timing roller and the anterior sheet transporting mechanism on the copying machine side and the sheet transporting mechanism and the drive mechanism on the base unit side produces differences in drive torque and inertia. These differences in torque and inertia likewise provide different stopping and starting characteristics.
Therefore, when the transfer sheet is re-transported after being temporarily stopped as previously described, a loop is produced in the transfer sheet between the sheet transporting mechanism of the copying machine and the sheet transporting mechanism of the base unit. This loop is caused by differences in the stopping characteristics of the temporary stopping interval.
The sheet transporting mechanism in the base unit and the sheet transporting mechanism in the copying machine are both driven simultaneously when the transfer sheet is re-transported. Therefore, when the transfer sheet is re-transported while in the previously described looped condition, wrinkling and jamming of the sheet may occur.
The amount of looping of the sheet will differ depending on the material and thickness and the like of the sheet, and depending on differences in the transporting characteristics at times. Accordingly, this looping condition cannot be corrected via a predetermined correction value.